Apparatus for producing electrostatic images



April'28,"1970 v GRoom Y Y 3,508,477

APPARATUS on PRODUCING :ELECTROSVIATIC IMAGES Y I Filed Dec. 6, 1967 I 3 Sheets-Sheet 1 FIG.

IMAGING DEVICE 1 v Elerrosrarip Image Forming nuflgrv Devi ce I I 1 a TONER APPLIOATOR DRIVE UNIT INVENTOR.

HAYES RICHARD GRCO, 111

h is ATTORNEYS I A l n 1,9705 I I I 3,508,477

v APPARATUS FOR PRODUCING ELECTROSTATIC "IMAGES 3 Sheets-Sheefc 2 FIG. 2

POWER SUPPLY INVENTOR. HAYES RICHARD mm, m

his ATTORNEYS *ij-Ar i il o. I 1.7.1! {3,508,471-

V'APPAI'KATYUS' FOR. PRCDIIUCYING ELECTROSTATIC IMAGES F iled Dec. e,'f19'67 3 Sheets-Sheet 5 I INVENTORL HAYES momma GROO, m

ATTORNEYS United States Patent 3,508,477 APPARATUS FOR PRODUCING ELECTROSTATIC IMAGES Hayes Richard Groo III, Stamford, Conn., assiguor to Columbia Broadcasting System, Inc., New York, N.Y., a corporation of New York Filed Dec. 6, 1967, Ser. No. 688,524 Int. Cl. G01 15/06 U.S. Cl. 95-1 Claims ABSTRACT OF THE DISCLOSURE Apparatus for producing electrostatic images comprises an evacuated glass envelope having a window for admitting photo information and a-multi-cathode including an array of spaced, parallel fine wire conductors hermetically sealed in the envelope wall generally opposite the Window. The conductors transect the envelope wall and have their inner and outer ends, respectively, exposed. The inner ends of the conductors have a deposit of photoemissive material. A transparent conductive coating on the inner surface of the window serves as an anode. A substrate on which an image is to be produced is brought into contact with the outer, exposed ends of the conductors of the multicathode, and, upon imaging photo information on the multi-cathode photoemitters, electrons are emitted to the anode, the electrons being supplied to the photoemitter BACKGROUND OF THE INVENTION This invention relates to electrostatic printing and, more particularly, to a novel and improved apparatus for producing an electrostatic image on a substrate.

An important type of printing technique, especially in the area of making facsimile copies, is that generally known as xerography. Xerography involves forming a latent electrostatic image of a master copy, such as by forming the image on a plate, drum or belt that is coated with a photosensitive material, giving the photosensitive surface a uniform electrostatic charge, and then selectively discharging the photosensitive surface to leave a latent electrostatic image by imaging the master on it. The surface is thereafter dusted with a toner material of opposite charge, the toner sticking only to those parts of the photosensitive surface that are of opposite charge to the toner. The toner is then transferred from the surface to a copy sheet and is fixed, generally by heat, to leave a permanent visable image on the copy sheet. Although this form of electrostatic copying provides good results, it requires relatively complicated and expensive equipment and suffers from the disadvantage of requiring relatively frequent replacement of the photosensitive plate, drum or belt. Moreover, the speed of operation is relatively slow.

A modification of the above-described type of equipment involves the use of special paper that is pre-coated with a photosensitive material, but is basically the same, except that the electrostatic image is produced directly on the paper. The equipment for carrying out this form of electrostatic copying process is less complicated and less expensive, but the paper is expensive, the quality of the copies is generally inferior, and the speed is relatively slow.

Another way of producing an electrostatic image on a substrate is by directly depositing electrons from an electron source that is controlled by photo information. In

one form this technique utilizes a cathode ray tube as the electron source, the electron beam scanning a screen that has a multiplicity of closely spaced conductors transecting it. The electron beam is controlled by a video signal, and as it scans the screen, the electrons are conducted through the conductors to a copy sheet brought into contact with the screen, thereby producing a surface charge on the copy sheet. This technique is susceptible of high speeds, but is expensive and, as a practical matter, is generally not well suited for direct copying, inasmuch as it requires relatively complex, expensive equipment for transforming the master copy into a video signal.

It has also been proposed to transfer an electrostatic image directly onto a copy sheet by means of a multiplicity of conductors in a matrix of non-conductive material, each of which is adapted to be coupled to one side of a source of electric potential by a photo-conductive ele ment. The copy sheet is interposed between the ends of the conductors and a backing plate connected to the other side of the source of electric potential. The master copy is imaged on the photoconductors, thereby selectively switching the voltage to those conductors upon which light impinges and creating an electric field across the copy sheet at each energized conductor. Among the disadvantages of this type of device are the high cost and difficulty of manufacturing the photo-conductor plate with sufficient density to produce a high resolution image and of providing suflicient conductivity in the photo-conductors to deliver effective charges to the copy paper but at the same time low enough to prevent breakdown voltages between the photoconductors.

SUMMARY OF THE INVENTION There is provided, in accordance with this invention, a novel and improved apparatus for producing electrostatic images on a substrate from photo information, such apparatus overcoming many of the disadvantages of previously known devices for this purpose and offering distinct advantages, such as the ability to produce images of very high resolution and to operate at very high speed. The apparatus of the invention can be manufactured at relatively low cost, has a relatively long life, and requires a minimum of maintenance or repair.

More particularly, the apparatus of the invention comprises an evacuated envelope of non-conductive material, such as glass, having a window for admitting photo information, a multi-cathode element composed of a multiplicity of substantially uniformly spaced, parallel conductors hermetically sealed in the wall of the envelope in a position to receive the photo information imaged through the window, and an anode. The conductors transect the envelope wall and have their inner and outer ends exposed on the inner and outer surfaces, respectively, of the wall.

On the inner end of each conductor of the multi-cathode is an element or deposit of a photoemissive material, and upon imaging the photo information on the photoemissive deposits, electrons are drawn from the substrate, such as a suitable paper, brought into contact with the outer ends of the conductors through the conductors and are emitted to the anode, thereby leaving an electrostatic latent image on the substrate.

The electrons emitted by the photoemitters are supplied either from a surface charge impressed on the copy paper or other substrate from a conductive backing plate behind the paper and coupled to a source of electric potential or from a previously deposited electrostatic charge impressed on the paper by suitable means, such as a corona discharge device, before the paper is brought into contact with the multi-cathode. In the former case an electric field is established across the paper between the backing plate and each element of the multi-cathode which has electrons drawn fromit by emission of electrons to the anode by its photoemitter. At each such point, which is in turn established by light irradiating the photoemitter, electrons are drawn from the surface of the paper, thereby leaving a positive charge. The remainder of the paper surface carries a neutral charge. This form of electrostatic image can be developed into a visible image by dusting with a negatively charged toner.

Where the copy substrate is given a negative electrostatic surface charge prior to being brought into contact with the multi-cathode, the charged surface of the paper itself constitutes a source of electrons for the photoemitters, the electrons being drawn off the surface of the copy substrate and emitted to the anode, from which they can be conducted away, as to ground. This form of the invention, in which the source of electrons is a precharged surface, can be used in combination with a backing plate to create an electric field, as above.

In one embodiment of the apparatus of the invention the evacuated envelope or tube is elongated to a length appropriate to cover the width of the materials to be copied and has a multi-cathode in the form of a narrow,

elongated band. Suitable mechanical equipment can be used to provide relative movement between the master, the copy paper, and the tube so that the image is scanned relatively across the tube and the copy scanned corre spondingly to generate progressively the electrostatic image.

In another form the tube is of full format size, that is, of a size adequate to accommodate a multi-cathode of a length and width corresponding to the maximum dimensions of the copies to be made. In this case the master, copy sheet and tube remain stationary during exposure of the copy, and suitable equipment can be provided to conduct the master and copy sheet to and from their exposure positions with a dwell time for exposure or formation of the image.

The apparatus of the invention offers a number of distinct advantages. For one thing, it is adapted to either direct or indirect copying from a visible master, either by directly imaging the master using a suitable lens or mirror optical system to focus the image on the photoemitters or by imaging another form of photo information on the photoemitters, such as the image of a video reproduction of the master or the image of a video character writer driven by a computer input. The apparatus is capable of high operating speeds. With a scanningtype system, high resolution copies are obtainable by virtue of the movement of the image across the multicathode, which preferably consists of rows of cathode elements, the elements of the several rows being staggered lengthwise of the tube. A similar effect can be obtained by imparting oscillatory motion of a suitable amplitude and direction to the tube of a full format system, thereby to produce a denser latent image. In either case, the density of the multi-cathode can be established to limit inter-element capacitance effects while still providing high resolution copies. Inasmuch as the electron emissivity of the multi-cathodes is a function of the radiant energy input of the photo information, tone gradients of good quality are obtained. The apparatus does not require any complex, expensive electronic units, such as those necessary for the cathode ray tube type image-forming devices. It can be constructed, operated and maintained at a reasonable cost, especially when its operating speed is taken into account.

.BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be made to the following description of exemplary embodiments, taken in conjunction with the figures of the accompanying drawings, in which:

FIG. 1 illustrates, in schematic form, an exemplary reproducing system according to the invention;

FIG. 2 is an end cross-sectional view, in generally schematic form, of one embodiment of a tube and an electron source for use in the system of FIG. 1 and also illustrating a lens, master copy, and copy sheet in operative positions;

FIG. 3 is an end cross-sectional similar to FIG. 2 of another embodiment of the invention which incorporates a modified way of impressing an electrostatic charge on the copy substrate;

FIGS. 4 and 5 are side and end views in cross-section, respectively, showing a scanning type tube appropriate for the system of FIG. 1; and

FIG. 6 is a plan view, and FIG. 7 an end view in section, of a full-format type tube.

DESCRIPTION OF EXEMPLARY EMBODIMENTS The reproducing system, according to the invention, in the form illustrated schematically in FIG. 1, employs master material in strip or web form supplied from a supply reel. The Web is taken oif the supply reel and moved to the right, as indicated by the arrow, by a driven take-up reel at a substantially constant speed. It is appropriately guided along its path of movement through an imaging device, such as that represented schematically in FIG. 1 by a light source and a lens, which images the master on an apparatus for forming an electrostatic image on a copy substrate, the copy substrate being in web form and being taken off a supply reel and wound onto a take-up reel. The copy web is conducted through the electrostatic image-forming device at the same speed as the master moves through the imaging device and is given an electrostatic surface charge corresponding to the image on the master. The copy web moves from the imageforming device to a toner applicator and fuser where a visible, charged toner is applied and fixed, thus completing the reproduction process.

The master and copy material feeding equipment, the imaging device and the toner applicator and fuser may be of various types and of various specific designs. Those elements of the apparatus, therefore, are not shown or described in detail, inasmuch as those skilled in the art can readily adopt known devices for the system of the invention.

FIG. 2 showsthe electrostatic image-forming part of the system of FIG. 1 on a larger scale. It includes an evacuated envelope or tube, designated generally by the reference numeral 10' in the drawing, of non-conductive material, preferably glass. The tube may have any ap propriate shape, as discussed further below. One wall 12 of the tube 10 has a window for admitting into the tube the image, which is represented in the figure by an arrowed line designated I, of a master copy or any suitable object to be copied (designated by the letter M) imaged on a multi-cathode located in the wall 14 of the tube generally opposite the window.

The multi-cathode of the tube 10, which is designated generally by the reference numeral 16, is composed of 1) a multiplicity of electrical conductors 18 (shown for greater clarity in greatly enlarged, exaggerated scale relative to the overall scale of the figure), each of which transects the wall of the tube and has its inner and outer ends exposed at the inner and outer surfaces, respectively, of the wall 14 and (2) an element or deposit of photoemissive material on the inner end of each conductor, the deposits being frequently referred to hereinafter as photoemitters and designated by the reference numeral 20 in the drawing.

The conductors 18 are fine wires or fibers of conductive material which are uniformly spaced from each other, are in parallel relation to each other and are aligned perpendicular to the major plane of the wall. They are hermetically sealed to the material of the wall, which constitutes a matrix for them.

A member or element appropriate for the multiconductor wall of the tube 10 is available under the trademark Multilead from Corning Glass Works, Industrial Bulb Sales Department, Corning, N.Y. It is available with a number of different conductor materials and sizes and a number of different spacings between the conductors. For use in the apparatus of the invention the conductor size and spacing will be selected with a view to the desired resolution of the electrostatic image. The Multilead material comes in sheet or strip form and can be incorporated into the tube 10 by a suitable glass fusion technique.

The photoemitters 20 may be deposited on the inner, exposed ends of the conductors by any suitable method of selective deposition, a number of which will be apparent to those skilled in the art. Advantageously, however, the method is as follows: the process begins with the multi-conductor element, such as Multilead, in strip or sheet form. The strip or sheet is first cleaned by suitable techniques to remove any foreign matter which might either provide false nucleating sites or contaminate subsequent depositions. The thoroughly cleaned plate is then selectively coated in the areas on which the photoemitters are later to be formed, i.e., primarily the inner ends of the conductors but also, if desired, small areas of the glass surrounding the conductor ends, with a prenucleating material having a latent heat of sublimation higher than those of the photoemitter materials. The prenucleating deposits constitute sites that are adapted to receive and retain the photoemitter materials deposited on them by a selective evaporation technique, as described below, and may themselves be deposited by any of a number of known mechanical, optical, chemical or other deposit techniques such as electrolytic plating using a suitable mask, electron beam bombardment of the desired sites to change their structure sufficiently for subsequent selective deposition, and so forth. For a photoemitter having a base of antimony, Nichrome is a suitable prenucleating material.

Following the deposit of the prenucleating material, the sheet or strip can be fabricated into the tube. The tube is fabricated with suitable internally located evaporators and sources of the photoemissive materials (not shown) so that the photoemitters can be evaporated onto the prenucleating prepared sites. The tube is then placed in a vacuum and baked out to remove all foreign gases and other matter deleterious to the performance and processing of the photoemitters. Next, the entire tube is heated to a temperature such that the re-evaporation rate of the photoemitter materials is equal or greater than the rate of evaporation from the evaporating devices so that upon vaporizaion of the materials that will make up the photoemitters, the vapor will condense only on the prepared sites.

More particularly, the tube is evacuated to l0- torr or less and, for example, with antimony as one element of the photoemissive compound, is heated to a temperature of 250 C. to 350 C. The antimony is then evaporated from the evaporating device, such as a filament, thereby forming a vapor in the tube. The vapor condenses onto the prenucleated sites, but does not condense on any other surfaces of the tube. The antimony evaporator is then cooled down, and another evaporator is heated to evolve another component of the photoemitter, such as cesium, from a compound containing it. The cesium diffuses into the previously deposited antimony and forms cesium antimonide, a good photoemitter. The process thus provides photoemitters only on the prenucleated sites, and in particular only on the ends of the conductors, and possibly, small areas surrounding them. Thus, each conductor carries a separate photoemitter. The tube can then be closed in a suitable manner.

Returning to FIG. 2 of the drawings, the tube also has an anode 22, preferably in the form of a suitable transparent conductive coating deposited on the inner surface of the window portion 12, which constitutes a collector at a uniform distance from the multi-cathode for electrons emitted by the photoemitters, as described below. The anode 22 is connected by a conductor 24 to one side of a source of electric potential (direct current), as represented in the drawing by a battery 26. The other side of the battery 26 or other source is connected by a conductor 28 to a conductive backing plate 30. The backing plate 30 is located adjacent the outside surface of a multi-cathode and on the opposite side of a copy paper C or other substrate on which the image is to be formed from the multi-cathode. The backing plate is preferably mounted so as to hold the paper against the multi-cathode in a manner affording contact of the paper surface with the exposed outer ends of the conductors 18.

Assuming that there is no light irradiating the photoemitters, there is no current flowing in the circuit and only a negligible electric field across the tube from the backing plate to the anode. However, upon imaging the master copy M on the multi-cathode 16, as represented by the image I, those photoemitters that receive light radiation from the image emit electrons to the anode in an amount proportionate to the magnitude of the light energy. The electrons emitted by the photoemitters are replaced by electrons drawn from the conductorassociated with the given photoemitter, causing the conductor to become positively charged With respect to the paper at the opposite end. As a result, electrons are drawn from the paper by the conductor until the potential of the conductor is the same as that of the surface of the paper immediately adjacent to it, thereby leaving the copy paper with a positive electrostatic surface charge at all such'points. The magnitude of the charge at any given point is a function of the light radiated onto the photoemitter, the electron emission of which gave rise to the charge. The copy paper with the electrostatic image impressed thereon is then further processed to tone and fix the image by any appropriate technique. It will be noted that the apparatus of FIG. 2 produces a negative copy, that is, each illuminated region of the image produces a corresponding charge which, in turn attracts the usual dark toner material to produce a dark region in the copy. Consequently, to produce a positive copy, optical or charge reversal techniques are required or else a light toner may be used with a dark copy paper.

FIG. 3 shows the modification of the apparatus of FIG. 2 of providing positive copies. In this arrangement, a suitable precharging device 32, such as one embodying a corona discharge element is utilized to charge the surface of the paper negatively prior to exposure. The remaining elements of the apparatus are the same as those of FIG. 2, as designated by the same reference numerals with a prime sufiix. As a further modification the conductive backing plate is omitted, and the electrons collected on the anode are conducted to ground. The precharging device impresses a uniform negative electrostatic charge on the surface of the copy sheet of a magnitude such as to be neutralized by maximum positive charging of the electrodes 18 by exposure to light. In a manner similar to that of the embodiment of FIG. 2, the imaging of photo information, such as the image of a master copy, causes electrons to be drawn from the points on the paper that are in contact with conductors of the multi-cathode from which, in turn, electrons are drawn and emitted by the associated photoemitter. Accordingly, the electrostatic image produced on the paper by the apparatus of FIG. 3 consists of negative charges at those points corresponding to the dark portion of the master, and a substantially neutral charge condition at those points corresponding to the light portions of the master. The latent electrostatic image can then be toned in an appropriate manner, as before.

FIGS. 4 and 5 show one form of the tube, which is designated generally by the reference numeral 100, which is adapted to scan the source of photo information, such as the master copy and progressively trace an electrostatic image of the photo information on a copy paper or other copy substrate. The tube 100 includes an evacuated glass envelope 102, a base 104 and a plurality of pins (e.g., 106 to 109), for coupling the elements of the tube to associated equipment. Some of the pins are connected to evaporators (not shown) in the tube for vaporizing the source materials of the photoemitter array of the multicathode 114 during the manufacture of the tube. One of the pins is connected to the anode 112.

The multi-cathode 114 of the tube 100 of FIGS. 4 and 5 is in the form of a relatively narrow band (say 0.20 inches wide) having a length equal or greater than the width of the copies to be handled by the apparatus. The multi-cathode elements are preferably arranged in lengthwise rows with the elements of each row staggered with those of other rows, thereby to provide high resolution images with a relatively less dense cathode array. The same result can be achieved with regular, uniform spacing of the cathode elements by cocking the axis of the tube relative to the direction of movement if the master and copy materials. The tube of FIGS. 4 and 5 is adapted to be used with suitable mechanisms for providing relative movement between the master copy, the copy paper and the tube, such that the image is scanned and the electrostatic image is progressively formed upon conjoint movement of the copy material with the image, relative to the tube. It is understood, of course, that the master and copy paper may be moved while the tube remains stationary, or vice versa.

FIGS. 6 and 7 show a form of the tube according to the invention which is adapted to produce the electrostatic image With the master arid copy sheet remaining stationary relative to the tube during exposure. The tube 200 of FIGS. 6 and 7 includes an evacuated glass envelope having a body 202 which is generally circular in plan (FIG. 7). Extending from a peripheral wall of the body 202 is a neck portion 204 to which a base 206 is fitted, the base having pins 208 to 211, for example, that serve the same purposes as the pins on the base in the embodiment of FIGS. 4 and 5. One side wall 202a of the envelope body 202 constitutes a window through which the photo information can be imaged on the multi-cathode 214 that constitutes the major portion of the opposite wall 202b of the body, the window and multi-cathode being of generally the same size and shape and being shown in dotted lines in FIG. 6. The inner surface of the wall 202a (the window) has a transparent electrically conductive coating 218 (FIG. 7) constituting the anode. A copy sheet, shown in phantom lines in FIG. 6 and designated C, is brought into stationary position in contact with the outer ends of the multi-cathode conductors and is exposed to the stationary image of the master focused on the photoemitter (not visible). Suitable equipment can be employed to move the master and copy to and from their exposure positions. Proper exposure times for forming the electrostatic image are obtained by the duration of the imaging of the photo information on the cathode, such as by timing a light source that illuminates the image. The light source is turned off (or the image otherwise blanked) except when the master and copy sheet are stopped at dwell for exposure so that the image will not be smeared. It may be advantageous to impart oscillatory rr'iotion to the tube relative to the master and copy during exposure of the image to, in effect, spread the image dots provided by each cathode element and provide high resolution without requiring a high density cathode array.

The embodiments of the invention described above and illustrated in the drawings are intended to be merely exemplary, and those skilled in the art will be able to make numerous variations and modifications of them Without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.

I claim:

1. Apparatus for producing electrostatic images on a substrate from a source of photo information comprising an evacuated envelope having a Window for receiving photo information representing the image to be reproduced; a multi-cathode in the envelope including a multiplicity of individual closely spaced electrical conductors transecting the envelope wall and having inner and outer ends exposed respectively inside and outside of the envelope wall, the inner conductor ends being positioned in the envelope to receive photo information imaged through the window, and an individual element of photo emissive material carried on the inner end of each conductor and adapted to emit electrons in response to the imaging of photo information thereon; an anode in the envelope for collecting electrons emitted by the photoemissive elements; means for projecting photo information onto the multi-cathode conductor photoemissive ends within the envelope; and means for supporting the substrate in contact with the exposed outer ends of the multicathode conductors.

2. Apparatus according to claim 1 including a conductive backing plate positioned adjacent the multi-cathode to receive a substrate therebetween, and a source of electric potential connected across the anode and the backing plate thereby to impress an electric field across the substrate in conjunction With the individual conductors of the multi-cathode.

3. Apparatus according to claim 1 including precharging means for creating a negative electrostatic surface charge on a substrate before it is brought into contact with the multi-cathode.

4. Apparatus according to claim 1 wherein the anode is a transparent conductive coating on the window. v

5. Apparatus according to claim 1 wherein the envelope material is glass, and the multi-cathode conductors are substantially uniformly spaced parallel fine wires hermetically sealed in the glass.

6. Apparatus according to claim 1 wherein the inner and outer ends of the conductors are located in proximity to the inner and outer surfaces of the envelope Wall and are supported and positioned by the envelope wall ma terial.

7. Apparatus for producing electrostatic images on a substrate from a source of photo information representing the images to be produced comprising an evacuated glass envelope having a Window for admitting the source of photo information into the envelope; a multi-cathode in the envelope including a multiplicity of substantially I uniformly spaced parallel fine wire conductors hermetically sealed in a part of the envelope wall generally opposite the window to receive the photo information, the conductors transecting the wall and having their inner and outer surfaces exposed respectively on the inner and outer surfaces of the envelope wall, and a deposit of photoemissive material on the inner end of each conductor; an anode in the envelope in the form of a transparent conductive coating on the inner surface of the window; and means for producing a negative electrostatic surface charge on the substrate before it is brought into contact with the multi-cathode, whereby the multi-cathode emits electrons to the anode in response to the photo information impressed thereon, thereby selectively to draw olf electrons from the surface charge on the substrate when the substrate is brought into contact with the multi-cathode and to leave a latent electrostatic image on the surface of the substrate corresponding to the photo information.

8. Apparatus according to claim 7 further comprising a conductive backing plate positioned closely adjacent, and on the opposite side of the substrate from, the multicathode, and a source of electric potential connected across the backing plate and the anode, thereby to impress an electric field across the substrate at any point thereon Where a conductor and its associated photoemissive element emits electrons to the anode.

9 10 9. Apparatus according to claim 7 wherein the en- References Cited velope is of full format size, the multi-cathode having a UN TED STATES PATENTS substantial width and length, whereby the tube is adapted 2,952,796 9/1960 Crevm to produce an electrostatic image with a source of photo 3,419,888 12/ 1968 Levy. information and a substrate on which the electrostatic 5 OTHER REFERENCES image is to be produced being substantially stationa R.C.A. TN No. 104; An Electronic Stylus TubePaul relative to the envelope' W. Kasemon, published May 12, 1958.

10. Apparatus according to claim 9 further comprising means for efiecting oscillatory motion of the tube rela- 10 NORTON ANSHER Pnmary Examiner tive to the copy substrate to increase the image density MCCORMICK, Assistant Examiner relative to the density of the multi-cathode conductor 1 array. 250-49.5 34674 

